The present disclosure relates generally to electronic memory technology, and more specifically to generating spin current for spin transfer torque magnetic random access memory (STT-MRAM) using an anomalous hall effect and/or polarized spin hall effect.
Electronic memory may be classified as either volatile or non-volatile. Volatile memory requires constant power to retain stored data, while non-volatile memory does not. A common memory found in computers is volatile random access memory (RAM), which provides fast read/write speeds and easy re-write capability. However, when system power is switched off, any information not copied from volatile RAM to a hard drive is lost. Although non-volatile memory does not require constant power to retain its stored data, it in general has lower read/write speeds and a relatively limited lifetime in comparison to volatile memory.
Magnetoresistive random access memory (MRAM) is a non-volatile memory that combines a magnetic device with standard silicon-based microelectronics to obtain the combined attributes of non-volatility, high-speed read/write operations, high read/write endurance and data retention. Data is stored in MRAM as magnetic states or characteristics (e.g., polarity or magnetic moment) instead of electric charges. In a typical configuration, each MRAM cell includes a transistor, a magnetic tunnel junction (MTJ) device for data storage, a bit line and a word line. In general, the MTJ's electrical resistance will be high or low based on the relative magnetic states of certain MTJ layers. Data is written to the MTJ by applying certain magnetic fields or charge currents to switch the magnetic states of the MTJ layers. Data is read by detecting the resistance of the MTJ. Using a magnetic state/characteristic for storage has two main benefits. First, unlike electric charge, magnetic state does not leak away with time, so the stored data remains even when system power is turned off. Second, switching magnetic states has no known wear-out mechanism.
The term “magnetoresistance” describes the effect whereby a change to certain magnetic states of the MTJ storage element results in a change to the MTJ resistance, hence the name “Magnetoresistive” RAM. A typical MTJ includes a fixed magnetic layer, a thin dielectric tunnel barrier and a free magnetic layer. The MTJ has a low resistance when the magnetic moment of its free layer is parallel to the magnetic moment of its fixed layer. Conversely, the MTJ has a high resistance when its free layer magnetic moment is oriented anti-parallel to its fixed layer magnetic moment. The MTJ can be read by activating its associated word line transistor, which switches current from a bit line through the MTJ. The MTJ resistance can be determined from the sensed current, which is itself based on the polarity of the free layer. Conventionally, if the fixed layer and free layer have the same polarity, the resistance is low and a “0” is read. If the fixed layer and free layer have opposite polarity, the resistance is higher and a “1” is read.
Spin Transfer Torque MRAM (STT-MRAM) uses electrons that have been spin-polarized to switch the magnetic state of the MTJ free layer. During the write operation, the spin-polarized electrons exert a torque on the free layer, which can switch the free layer magnetic state. Thus, the required amount of STT-MRAM writing current depends on how efficiently spin polarization is generated. Additionally, designs that keep write currents small (e.g., Ic<25 micro-ampere) are important to improving STT-MRAM scalability. This is because a larger switching current would require a larger transistor area, which would inhibit the ability to scale up STT-MRAM density.
A typical STT-MRAM configuration uses a thin film (e.g., spin filter) to spin polarize electrons as they pass through the film. The Spin Hall Effect (SHE) has been proposed as an alternative approach to generating spin polarized electrons and their resultant spin current. For example, a non-magnetic SHE material could, in theory, be used to generate spin current in the transverse direction while a charge current flows longitudinally. The generated spin current can be injected into an adjacent layer to switch the adjacent layer's magnetic state. For each electron that passes through the SHE material there can be more than one spin injected into the adjacent layer, which can result in a high rate of efficiency in transferring the spin angular momentum.
However, it can be difficult to combine SHE material with an MTJ. For example, as described above, the tunneling current through the MTJ and the current that provides the SHE flow in different paths and in different directions (e.g., orthogonal to one another). In conventional designs, three terminal devices are used wherein the tunneling current across the MTJ and the current that generates the SHE torque are applied across different pairs of terminals. These and other issues are addressed by a pending U.S. patent application, assigned to the assignee of the present disclosure and filed Mar. 15, 2013, entitled “SPIN HALL EFFECT ASSISTED SPIN TRANSFER TORQUE MAGNETIC RANDOM ACCESS MEMORY,” by John K. DeBrosse, Luqiao Liu and Daniel Worledge, having U.S. application Ser. No. 13/835,355, and expressly incorporated by reference herein.
Other designs that attempt to use SHE material to switch a free layer magnetic state generally deal with MTJs having in-plane magnetic anisotropy, which means that the activation energy cannot be very high for small junctions (e.g., L. Liu et al., “Spin-Torque Switching with the Giant Spin Hall Effect of Tantalum,” Science, Volume 336, No. 6081, 4 May 2012, pp. 555-558). Although designs have been proposed for using SHE material to switch a free layer magnetic state having perpendicular magnetic anisotropy (PMA), these designs suffer from significant inefficiencies (e.g., I. M. Miron, K. Garello, G. Gaudin, P.-J. Zermatten, M. V. Costache, S. Auffret, S. Bandiera, B. Rodmacq, A. Schuhl and P. Gambardella, Nature (London) 476, 189 (2011); and Luqiao Liu, O. J. Lee, T. J. Gudmundsen, D. C. Ralph and R. A. Buhrman, Phys. Rev. Lett. 109, 096602 (2012)).
Accordingly, there still is a need for non-volatile memory that efficiently lowers the switching current for STT-MRAM configurations having PMA.